1. Field of the Invention
The present invention relates generally to non-integral fan platforms for gas turbine engine fan assemblies and, more specifically, to such platforms and a fan assembly having fan blades with circular arc dovetail roots that mate in circular arc dovetail slots in a disk of a fan rotor.
2. Discussion of the Background Art
A turbofan gas turbine engine used for powering an aircraft in flight includes a fan assembly having a plurality of circumferentially spaced apart fan blades extending radially outwardly from a rotor disk. Ambient airflow is channeled between the blades and pressurized thereby for generating thrust for powering the aircraft in flight. The fan assembly typically includes a plurality of circumferentially spaced apart fan blades each having a dovetail root disposed in a complementary, axially extending dovetail groove or slot in a perimeter or rim of a rotor disk. The dovetail grooves are defined by dovetail posts and are complementary in configuration with the blade dovetail roots for radially retaining the blades to the rotor disk. The blades are also axially retained in the rotor disk to prevent axial movement of the blades in the upstream and downstream directions. A spinner is mounted to a front end of the fan assembly to provide smooth airflow into the fan.
A radially inner flowpath boundary for the airflow channeled between the blades is provided typically by integral or non-integral platforms at the blade roots which circumferentially abut each other between adjacent fan blades. There are often reasons for increasing the airflow through the fan blades to increase the thrust without increasing or holding to a limit of a tip of the fan blade. An inner flowpath boundary, often referred to as the hub, is moved radially inwardly from an existing engine design or the engine is originally designed to have, what is referred to as, a low radius hub. Conventionally, a radial transition portion is a transition from a curved blade section at the flowpath to a straight shank at the top of the dovetail. Due to the low inner diameter of the flow path, the radial transition portion of the fan blade from the aerodynamic or curved portion of the blade to the dovetail root is significantly shortened. It is desirable to have a fan disk assembly for a low radius hub design which incorporates a circular arc dovetail root and slot to ensure an adequate footprint and load path into the disk. The circular arc dovetail is arc shaped as disclosed in U.S. Pat. No. 5,123,813 which discloses circular arc dovetail slots and roots having centerlines in the form of arcs of a circle when viewing the disk radially.
It is desirable to have a platform that incorporates features to help provide a low radius hub design which incorporates a circular arc dovetail root and slot. Deflection of the platform is conventionally prevented by generally axially extending stiffening features such as rails. These rails interfere with the disk at the forward edge of the platform and cause the radial height of the platform to be increased at the forward end, which is contrary to the design objectives of the low radius hub.
A non-integral gas turbine engine fan blade platform for circumferential disposition between fan blades, the platform includes a platform wall with a radially outer surface which faces radially outwardly and a radially inner surface which faces radially inwardly. The platform wall is sloped with respect to a centerline with an increasing radius of the outer surface in the axially aft direction. Forward, mid, and aft mounting lugs depend radially inwardly from the platform wall, a wedge shaped platform bumper depends radially inwardly from the inner surface of the platform wall, and the wedge is axially located forward of the mid mounting lug. Circumferentially curved forward stiffening ribs extend axially from the mid mounting lug to a forward edge of the platform bumper about where the platform bumper begins to depend radially inwardly from the inner surface of the platform wall.
An exemplary embodiment of the invention further has the forward stiffening ribs tapered down to the inner surface of the platform such that at any axial location, a first height of the forward stiffening ribs is less than a second height of the platform bumper along an axially extending bumper length. Circumferentially curved aft stiffening ribs extend between the mid and aft mounting lugs. The platform wall further includes a rectangularly shaped forward portion and a circumferentially curved aft portion. The circumferentially curved aft portion has pressure and suction side edges that have pressure and suction side airfoil shapes, respectively.
The mid and aft mounting lugs have axially extending mid and aft holes respectively therethrough, bushings disposed within the mid and aft holes, and the bushings are made of a harder material than that of the lugs. The bushings are pressed into the holes with an interference on the order of 1.5-2.5 mils on diameter.